As the result of positive and negative selection, T cell differentiation in the thymus generates a repertoire of mature T cells that is tolerant to self-antigens but strongly responsive to foreign antigens. Via apoptosis induction, negative selection prevents the production of potentially dangerous T cells with high avidity for self-antigens. As the result of negative selection (central tolerance), most T cells released from the thymus are unresponsive to self-antigens. However, because self-antigens may not be represented in the thymus, some auto-reactive T cells escape from the thymus and are controlled by peripheral mechanisms of tolerance such as immunoregulation and anergy induction. The development of autoimmune disease is thought to reflect abnormalities in central tolerance, peripheral tolerance, or both. The NOD strain of mice, which spontaneously develop type I diabetes, is a useful model for studying defective self- tolerance induction. Experiments are proposed to examine whether NOD mice show defects in negative selection. Using both in vitro and in vivo models of negative selection, purified thymocyte subsets from NOD and other strains will be characterized with regard to their susceptibility to T cell receptor (TCR) induced apoptosis, expression of molecules affecting apoptosis, intracellular signaling pathways and sensitivity to cytokines.